Originally Posted by
Thomas McCurnin
if the Romex or 12ga wire in the old box is directly connected to a receptacle, then I would not stack the new 12 ga wire on top of the old wires in the receptacle and torque down the screws for two reasons: (1) You're essentially using the receptacle as large un-insulated wire nut; (2) I'm not sure the screw in the receptacle will back off enough to allow two sets of 12ga wire underneath it. It is designed to accommodate only one set of wires, not two.
Better to add a set of the short 3" pig tails to the connection for the old receptacle connected by wire nuts. Then make the connection for the new Romex or 12 ga wire to those wire nuts. The connection is more secure and insulated.
The reason for not having two wires under one screw is it is easy for the wires to be placed with one crossing the other. This can cause an eventual failure of one or both of the wires.
Heating and cooling cycles can also increase the chances of the binding screws becoming loose.
Some junctions are made with multiple washers to separate the wires for stacking. These are usually for use in low power circuits.
jtk
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